Rising dissolved organic carbon concentrations in coastal waters of northwestern Borneo related to tropical peatland conversion

Southeast Asia’s peatlands are considered a globally important source of terrigenous dissolved organic carbon (DOC) to the ocean. Human disturbance has probably increased peatland DOC fluxes, but the lack of monitoring has precluded a robust demonstration of such a regional-scale impact. Here, we use a time series of satellite ocean color data from northwestern Borneo to show that DOC concentrations in coastal waters have increased between 2002 and 2021 by 0.31 μmol liter−1 year−1 (95% confidence interval, 0.18 to 0.44 μmol liter−1 year−1). We show that this was caused by a ≥30% increase in the concentration of terrigenous DOC and coincided with the conversion of 69% of regional peatland area to nonforest land cover, suggesting that peatland conversion has substantially increased DOC fluxes to the sea. This rise in DOC concentration has also increased the underwater light absorption by dissolved organic matter, which may affect marine productivity by altering underwater light availability.

. Long-term (2002Long-term ( -2021 monthly mean rainfall across the study region, derived from the Integrated Multi-satellitE Retrievals for GPM (IMERG; see Methods). Highest rainfall is seen during the November-February northeast monsoon period. Figure S3. Long-term (2002Long-term ( -2021 monthly mean absorption coefficient of CDOM at 440 nm (aCDOM) across the study region. During the wetter northeast monsoon (Nov-Feb), CDOM absorption in coastal waters is increased, and high-CDOM waters extend further offshore, consistent with the CDOM being predominantly terrigenous. Figure S4. Long-term (2002Long-term ( -2021 monthly mean dissolved organic carbon (DOC) concentration across the study region. During the wetter northeast monsoon period (Nov-Feb), high concentrations of DOC extend further offshore, consistent with a predominant terrestrial source. Note that our remote sensing method underestimates DOC concentrations in optically clear waters far from shore corresponding to optical water type 1 (see Fig. S1 and Methods).

Figure S5.
Long-term (2002-2021) monthly mean total suspended matter (TSM) concentration across the study region. During the wetter northeast monsoon period (Nov-Feb), TSM concentrations are elevated in coastal waters, indicative of greater river run-off during this period.

Figure S6
Long-term (2002-2021) monthly mean of the source index (γ0) of colored dissolved organic matter (CDOM) across the study region. Values of γ0 ≥0.5 reflect increasing contributions of terrigenous CDOM, and the spatial pattern indicates that CDOM in coastal waters is predominantly terrigenous in all months. During the wetter northeast monsoon period (Nov-Feb), high values of γ0 extend further offshore, indicative of greater land-ocean CDOM flux during these months. Spatial distribution of the mean annual colored dissolved organic matter (CDOM) absorption, showing increases in CDOM in coastal waters from 2003 to 2020. As for mean annual DOC concentrations (c.f. Fig. 2), rising CDOM absorption is seen especially adjacent to the main areas of peatland that have been converted to non-forest cover. Land-cover classification follows the Nusantara Atlas (see Methods).

Fig. S8
Time series of monthly (grey) and annual (blue) mean (a) the CDOM source index γ0, and (b) DOC-specific CDOM absorption at 440 nm (i.e. the CDOM:DOC ratio), both across the region of coastal water. Red trend line in (a) shows a statistically significant increase in monthly mean γ0 across coastal waters. Spectral slope of colored dissolved organic matter (CDOM) from 275-295 nm, plotted against the DOC-specific CDOM absorption (a*y440, which is the ratio of CDOM absorption to DOC concentration) at 440 nm, for the in-situ data we measured in rivers and coastal waters of northwestern Borneo (Refs. (35,38) in main manuscript). High values of a*y440 (≥0.005) are always associated with low values of the spectral slope S275-295 (≤0.015), which shows that dissolved organic matter with high a*y440 values has a terrigenous origin in our study region.  Fig. 4 in the main manuscript. The high trends for CDOM, DOC, and γ0 seen in coastal waters adjacent to the main peatland regions were statistically significant (p<0.05), while trends for TSM in coastal waters were largely non-significant.

Fig. S11.
Time series of monthly (grey circles) and annual (blue dots) average cloud cover across the study domain. Cloud cover is permanently high in this equatorial location, but there was no significant trend in cloud cover over the whole period. For the increase in DOC and CDOM concentration to be explicable by a reduction in the photodegradation rate, the cloud cover would need to have increased over time, which was not observed.   (440)), and TSM concentration for each station in our spectral library. Standard deviations were calculated with a Monte Carlo simulation by generating 5000 random spectra for each station based on the actual remote sensing reflectance (Rrs) spectrum of that station and the estimated uncertainty in Rrs for each wavelength. The random spectra were then passed to our optical model, and the standard deviation of the resulting estimated concentrations was calculated. Standard deviations are plotted here against the actual concentration from the optical model for the Rrs spectrum associated with that station. Solid and dotted horizontal lines indicate the mean and median standard deviation, respectively, of all coastal water stations (optical water types 2 and 3).   Table S1.
Parameter estimates and statistical significance of multiple linear regressions of annual mean dissolved organic carbon (DOC, in µmol l -1 ), colored dissolved organic matter (CDOM, as absorption coefficient at 440 nm, in m -1 ), and CDOM source index γ0 across coastal waters of our study region with cumulative peatland conversion and annual rainfall over land.  Table S2.

R2
Comparison statistics of satellite-derived aerosol optical depth (AOD) and remote sensing reflectance (Rrs) obtained using the NASA, MUMM, and SWIR atmospheric correction procedures with in-situ measurements. Numbers from 412 to 667 for Rrs indicate the wavelength in nm of the measurement.